The present invention relates to processing systems for processing workpieces, and more particularly, to ion implantation systems for implanting workpieces.
Ion implantation has become a standard, commercially accepted technique for introducing conductivity-altering dopants into a workpiece, such as a semiconductor wafer or thin film deposition on a glass substrate, in a controlled and rapid manner. Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. This beam is directed at the surface of the workpiece. Typically, the energetic ions of the ion beam penetrate into the bulk of the workpiece and are embedded into the crystalline lattice of the material to form a region of desired conductivity. This ion implantation process is typically performed in a high vacuum, gas-tight process chamber which encases a workpiece handling assembly, a workpiece support assembly, and the ion source. This high vacuum environment prevents dispersion of the ion beam by collisions with gas molecules and also minimizes the risk of contamination of the workpiece by airborne particulates.
The process chamber is typically coupled via a valve assembly with a processing end station. The end station can include an intermediate loadlock chamber or pressure lock which can be pumped down from atmospheric pressure by a vacuum pumping system. The chamber is selectively closed at a downstream end by the valve assembly, which selectively places the loadlock chamber in fluid communication with the process chamber. The loadlock chamber is also coupled at an opposite end to an upstream valve assembly. The end station also includes an end effector which transfers workpieces from one or more workpiece cassettes, through the upstream valve assembly, and into the chamber. Once a workpiece has been loaded into the intermediate chamber by the end effector, the chamber is evacuated via the pumping system to a high vacuum condition compatible with the process chamber. The valve assembly at the downstream end of the intermediate chamber then opens and the workpiece handling assembly mounted within the process chamber removes the workpiece from the intermediate chamber and transfers the workpiece to the support assembly, which supports the workpiece during processing. For example, a loading arm of the workpiece handling assembly removes the workpiece from the intermediate chamber and places it on a platen of the workpiece support structure. The workpiece support then moves the workpiece in the scanning direction past the operating ion source, which implants the workpiece.
Prior to implantation of the workpiece, each workpiece can be coated with a masking layer such as a photoresist layer to create a selected pattern on the face of the workpiece according to conventional photolithographic techniques. According to a conventional practice, the photoresist layer is removed in areas where ion implantation is to take place and remains as a mask over the remainder of the workpiece face.
During the implantation process, the ion beam implants those areas of the workpiece surface where the photoresist has been removed to produce the desired doping characteristics. For the remaining photoresist covered regions, the ions of the ion beam (which form an ion shower) penetrate the photoresist and undergo collisions with electrons and nuclei of the photoresist material and eventually come to rest. Since the photoresist is usually made of an organic polymer, the energetic ions cleave the hydrocarbon chains of the polymeric material as the ions travel therethrough. Consequently, the photoresist outgasses hydrogen, water vapor and other residue from the photoresist.
The outgassed residue condenses everywhere within line of sight of the photoresist coated surface. This residue, which is an insulating material, collects on the ion source, and specifically on the extraction electrode assembly of the ion source. Due to the relatively large dimensions of the ion beam extracted from the source, it is impractical to prevent overcoating of the electrodes. After prolonged exposure of the electrodes to the outgassed residue, it becomes necessary to dismantle the ion beam assembly in order to clean the electrodes. This accordingly results in down-time for the implantation system, thus decreasing the throughput of the ion implantation system, while concomitantly increasing the costs of operation.
Hence, there exists a need in the art for improved ion implantation systems that reduce the amount of outgassed photoresist or impede the coating of the extraction electrode assembly by the outgassed photoresist.